It is common practice that instrumentation panels adopted for both industrial and educational purposes incorporate frontal displays such as a voltage meter, ammeter, tachometer, and temperature indicators, or the like, to show the operational status of a machine at work, whereby the operator can take control of the overall running conditions of the machine in question, and can respond to switch off the power source as soon as he detects any abnormal condition in the course of operation, so as to maintain the machine in a safe and normal operating condition at all times.